How to convey that admonitions are side-comments that may be skipped (in most cases)

There is this weird contradiction that most of the time:
- we use admonitions for side-comments
- because they are colored brings more attention to them than they should. One example off the top of my head that we have in plenty of places:

Ideas of the top of my head:
- technically simpler but less visually appealing: can some admonitions advertising more easily that they are side-comments maybe with a less visible color e.g .greyish?
- technically harder but more visually appealing: is there a way to have a foldable admonition so that you see the title only and you can click on it to see more? Can we support a similar thing inside notebooks through our py-to-ipynb conversion logic in https://github.com/INRIA/scikit-learn-mooc/blob/bc157356571cddc9de6fba978e9446cecee1a0da/build_tools/convert-python-script-to-notebook.py#L120

Minor comments:
- I am pretty sure sometimes (quite rarely though I would say) we use admonitions as way to highlight that something is important.
- I guess there are more things than admonitions, like footnotes or sidenotes but supporting similar things in notebooks seems to hard
- does O'Reilly do something particular for this kind of stuff and his different notes to say "feel free to skip this" while still avoiding to disrupt the flow